Test motivation has been suggested to strongly influence low-stakes intelligence scores, with for instance, a recent meta-analysis of monetary incentive effects suggesting an average 9.6 IQ point impact (d = 0.64). Effects of such magnitude would have important implications for the predictive validity of intelligence tests. We report six studies (N = 4208) investigating the association and potential causal link of effort on cognitive performance. In three tests of the association of motivation with cognitive test scores we find a positive, but modest linear association of scores with reported effort (N = 3007: r ~ 0.28). In three randomized control tests of the effects of monetary incentive on test scores (total N = 1201), incentive effects were statistically non-significant in each study, showed no dose dependency, and jointly indicated an effect one quarter the size previously estimated (d = 0.166). These results suggest that, in neurotypical adults, individual differences in test motivation have, on average, a negligible influence on intelligence test performance. (≈ 2.5 IQ points). The association between test motivation and test performance likely partly reflects differences in ability, and subjective effort partly reflects outcome expectations.